Magnets and Magnetic Fields

Name______KEY______Date_5/23/11 Partner(s)______

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Investigation 12 Magnets and Magnetic Fields

I. Magnetic materials

A. Investigate the objects that you have been given (magnets, metals: aluminum and steel, cork, plastic, wood, rubber). Place the magnets in class 1. Separate the remaining objects into three classes based on their interactions with the magnets and each other.

1. List the objects in each of your classes.

Class 1 Class 2 Class 3 Magnets Interacts with Magnets No Interaction with Magnets

wood rod bar shape aluminum bar disk shape steel bar horseshoe shape small iron keepers copper rod etc. brass rod plastic

2. Fill out the table below with a word or two describing the interaction between members of the same and different classes, i.e., do they attract, repel, attract and repel, or is there no interaction?

Table of Interactions Class 1 Class 2 Class 3

Class 1 attract and repel attract no interaction

Class 2 no interaction no interaction

Class 3 no interaction

3. Are all metals in the same class? Do they all interact with magnets?

Not all metals interact with magnets. The only metal that does is steel.

B. Obtain a permanent magnet and an object that is attracted to the magnet but not repelled. Imagine that you do not know which object is the magnet. Using only these two objects, find a way to determine which object is the permanent magnet. (Hint: Are there parts on either object that do not interact as strongly as other parts?

If the textured bar is the magnet, then is A there will be a weak attraction between the rods. In B, there will be a strong attraction. Do you know why? A B C. The parts of a permanent magnet that interact most strongly with other materials are called the poles of the magnet.

How many magnetic poles does each of your magnets have? Explain how you found them.

Two poles – they represent the positions on the magnet that exhibit strong magnetic effects.

How many different types of poles do you have evidence for so far?

Only two.

II. Magnetic fields

We have observed that magnets interact even when they are not in direct contact. In electrostatics

we used the idea of an electric field to account for the interaction between charges that were

separated from one another. With magnetic interactions, we similarly define a magnetic field.

A. A compass is included in your box.

1. A compass is a small bar magnet that can rotate freely around a vertical axis. Being a bar magnet, it can interact with other magnets, and being small, it does not adversely affect the magnet with which it is interacting.

2. Move the compass far away from all other objects. Shake the compass. Does the needle behave as if it is in a magnetic field? How do you know?

After the needle comes to rest, it always ends up pointing in the same direction.

We can account for the behavior of the compass needle by supposing that it interacts with the earth and that the earth belongs to one of the categories from section I. To which class of objects from section I do your observations suggest the earth belongs? How you can tell?

The earth can be considered to be a member of Class I. It both attracts and repels other magnets e.g., compasses.

3. We define the north (seeking) pole of a magnet as the end that points toward the arctic region of the earth when the magnet is free to rotate and is not interacting with other nearby objects.

On the basis of this definition and assuming that the earth is a magnet, is the geographic north pole of the earth the north-seeking pole or the south-seeking pole of a magnet? Explain.

2 The geographic north pole of the earth acts like a magnetic south pole, because the north seeking ends of compasses and other magnets point toward the geographic north pole. They point in that direction because we know that unlike poles attract.

3 B. Magnetic field around a bar magnet.

If we were to break our bar magnet in half, we know that each half would become a new magnet with its own north and south poles. If we were to break the two halves in half, then each of those pieces would also become a new magnet. We could continue to smaller and smaller pieces to form many small magnets. This process can be reversed and a bar magnet can be built by combining smaller magnets to form a larger magnet.

Obtain some small magnets and stack them north-to-south until you have a bar magnet about the length shown below. Place your bar magnet on the outline shown on this page. Now use the compass to trace lines of force of the magnetic field surrounding the bar magnet. Include enough lines until you have a good feeling for the shape of the magnetic field. Be sure to indicate the direction of the magnetic field.

4 C. The magnetic field of the earth.

The magnetic field surrounding the earth is produced by convection currents in the outer core of the earth in combination with the rotation of the earth. The shape of the field, however, is very much like that of a bar magnet, and so one can imagine a bar magnet in the earth producing the field.

Below is a circle representing the earth with the geographic north and south poles shown. Place a small bar magnet (about four small magnets) in the outline drawn at the center of the earth. Use the compass to trace lines of force of the magnetic field surrounding the earth. Include enough lines until you have a good feeling for the shape of the magnetic field. Indicate the position of the magnetic north and south poles of the earth, the magnetic north and south poles of the bar magnet in the earth, and the direction of the magnetic field.

Geographic North Pole

Magnetic North Pole of the Earth

Magnetic South Pole of the Earth Geographic South Pole